The present invention relates to a mixer for use in agitating fluid in a pressure vessel. In particular, it relates to a magnetically driven mixer that eliminates the need for a seal around a mechanical drive shaft as is required for a mechanical mixer.
The use of magnetically driven mixers is known in the art. Magnetically driven mixers generally employ a rotating magnetic field located external to a mixing vessel, and a rotating agitator within the vessel that is coupled to the magnetic driver by one or more magnets located on, or embedded within the body of the agitator. Because such an agitator is magnetically coupled to its driver rather than being mechanically coupled, a sanitary environment can be maintained within the vessel without the need for packed or mechanical seals, such as would be required around a drive shaft that would penetrate the vessel if a mechanically coupled agitator were used. By eliminating such seals, leakage into or out of the vessel due to the agitator linkage can be eliminated.
Furthermore, with growing environmental concerns and stricter environmental regulations, magnetic mixers are gaining importance in the chemical industry in general. By eliminating the packed or mechanical seals associated with the mixer of an agitated vessel, fugitive emissions are eliminated as well as the possibility of releasing harmful or noxious fumes due to a seal failure.
Generally, in the prior art magnetically driven mixers, the agitator is held against a bearing surface by the same magnetic force that is used to drive the agitator. By its nature, such a system generates a thrust force against the bearing surface which can lead to increased friction as well as the generation of particles within the fluid being mixed due to the friction between the bearing surfaces. In many, if not most applications, the generation of particles is undesirable. Therefore, any reduction in the frictional forces between the associated bearing surfaces or other parts that contact one another is highly desirable.